There seem to be four quarks involved, but nobody's sure how they're linked.

Two different accelerators have found evidence for a particle that appears to contain four quarks, according to papers published in Physical Review Letters. Although particles with two and three quarks are common, this would be the first time that something containing four quarks has been spotted. Depending on the precise nature of the interactions among the quarks, this could be a discovery that keeps the theoreticians very busy.

With the discovery of the Higgs boson, the predicted collection of fundamental particles was complete. But one family of these fundamental particles—the quarks—combine with gluons to make more complex particles called hadrons and mesons. Hadrons include the proton and neutron, and they are formed by combinations of three quarks. Mesons, which are unstable, are comprised of pairs of quarks.

Having only two quarks would seem to make mesons fairly simple when actually they're anything but. There are three families of quarks, each with a particle and antiparticle, and mesons can consist of any combination of these. They also create some of the more amusing nomenclature in physics, with mesons involving a strange quark being referred to as strangeonium, and those with a bottom quark as bottomonium.

Earlier collider experiments had suggested that the presence of a meson with two bottom quarks might be associated with a heavier particle of unknown properties. So, two different teams, one working at a collider in Japan, the other at one in Beijing, decided to look at whether the same was true with charmonium. (Both colliders are electron-positron colliders, which have many advantages, despite their relatively low energies.)

So, the teams looked at events that included a J/ψ meson, which is a single particle (with two names, since two groups announced its discovery simultaneously) that is composed of a charm quark and a charm antiquark. To do this, they scanned the data sets generated by two different detectors: BES III in Beijing, and Belle at the KEK facility in Japan. The researchers pulled out those events that included a J/ψ and a pair of π particles (another type of meson), with the J/ψ being spotted due to its decay either into an electron and positron, or a muon and antimuon. With those in hand, they searched for indications that the J/ψ was the product of the decay of a heavier particle. (They do this by looking at what's called the "structure of the mass spectrum").

Both teams found something at 3.9GeV, which they're terming Zc(3900) due to its apparent mass of 3900MeV. But its presence is coupled to the appearance of charged π particles, which suggests that the new particle itself is charged. This means that it is probably comprised of four quarks. And, as mentioned above, particles with four quarks have not been previously detected.

The results have a statistical significance well above that required to count for discovery in particle physics, and the fact that there seems to be a similar heavy particle for bottom quarks suggests that this may be a common feature for all the quarks. There's also nothing in particular that rules out four-quark particles but we've gotten pretty deep into the era of particle physics without ever detecting one, so the results are surprising.

Most of the debate, however, seems to focus on how exactly could four quarks combine. One option is that they combine in the same way that two quarks combine with gluons to make mesons and three combine with gluons to form hadrons. The alternative would be what some reports are calling a "meson molecule," where a pair of two-quark mesons are held together by an attractive force. The problem with the latter option, as noted by Nature News, is that the molecule should be less stable than its constituent mesons. But the detectors see no sign of it splitting apart before it decays.

Given the clear method for spotting the Zc(3900) laid out by these papers, it should be easy for anybody to comb through their data and look for similar events, which may shed some more light on the particle's properties. And, in the mean time, it's a safe bet that theorists will be looking carefully at various forms of four-quark particles (molecule and otherwise) to see what sort of predictions they could make about the particle's behavior.

96 Reader Comments

With the discovery of the Higgs boson, the predicted collection of fundamental particles was complete.

I hate that characterization of the current state of particle physics. It's only sort of true, and it's misleading to the layman.

The standard model predicts 17 particles. We have now actually seen evidence of all 17 in detectors. But we know the standard model is incomplete. We're pretty much 99.999% sure there must be other particles, only we don't really know what we're looking for. (The current dark matter candidate, referred to as the WIMP, isn't one of those 17 identified particles, for example.) That contrasts with the search for the Higgs, for instance, in that we had a pretty good idea of what the Higgs would look like, we just didn't know exactly at what energy level we'd find that signature.

The quote makes it sound like we've detected all of the particles we ever predict we'll detect. Which really isn't true. A better characterization would be that "with the discovery of the Higgs boson, all of the particles that are well-described by the existing theory have been detected, but other particles are hypothesized."

The results have a statistical significance well above that required to count for discovery in particle physics,

That is arguable. There used to be, could still be, a CERN page that went through this.

In other cases, excepting biology/medicine which has a lot of individuality so 1-2 sigma may be all they get, we would want 3 sigma for hypothesis testing of theories. But we can see anything from 5 sigma to 7-9 (in astronomy) for hypothesis testing of ad hoc observations (against a random null hypothesis).

Seeing that 2 sigma is added to make up for the look-elsewhere-effect (i.e. not knowing the mass of a particle so looking at a lot of potential bumps), the CERN page suggested 5 sigma for theory testing but 7 sigma for isolated observations.

I think from the paper this is the latter. "Motivated by the striking observations of charged charmonium-like [4, 5] and bottomonium-like states [6], we investigate the existence of similar states as intermediate resonances in Y (4260) → π +π− J/ψ decays." [ http://arxiv.org/pdf/1304.0121v2.pdf ]

And since a 4 quark, or even a 2 meson, resonance would be unexpected, we should perhaps look for an additional 2 sigma, getting to astronomy's 9 sigma for "extraordinary claims".

Don't get me wrong, physicists will still use this results to investigate further. But we should be aware of the problem facing widespread acceptance.

Quote:

Hadrons include the proton and neutron, and they are formed by combinations of three quarks. Mesons, which are unstable, are comprised of pairs of quarks.

I think this model of long-lived composite particles died in the 60's.This is what particle physicist Matt Strassler says:

"You may have heard that a proton is made from three quarks. Indeed here are several pages that say so. This is a lie — a white lie, but a big one. In fact there are zillions of gluons, antiquarks, and quarks in a proton. The standard shorthand, “the proton is made from two up quarks and one down quark”, is really a statement that the proton has two more up quarks than up antiquarks, and one more down quark than down antiquarks. To make the glib shorthand correct you need to add the phrase “plus zillions of gluons and zillions of quark-antiquark pairs.” Without this phrase, one’s view of the proton is so simplistic that it is not possible to understand the LHC at all."

"It isn’t so easy to make predictions for collisions of objects that you can’t characterize in a simple way. But fortunately, starting back in the 1970s, following ideas of Bjorken from the late 1960s, theoretical physicists found a relatively simple and workable technique. Still, the technique only works to a certain extent, typically only accurate to ten percent or so (though occasionally better.) For this and several other reasons, the reliability of our calculations at the LHC is always somewhat limited."

So there is a content of "net" (readily visible) quarks, but most of the energy and so mass in a proton comes from pairs of relativistic quarks and gluons and their anti-particles moving at relativistic speeds.

And those bulk particles are created and annihilated all the time, though from Strassler's other writings I don't think they should be confused with virtual particles. Virtual particles wouldn't have real mass and energies, right?

With the discovery of the Higgs boson, the predicted collection of fundamental particles was complete.

I hate that characterization of the current state of particle physics. It's only sort of true, and it's misleading to the layman.

The standard model predicts 17 particles. We have now actually seen evidence of all 17 in detectors. But we know the standard model is incomplete. We're pretty much 99.999% sure there must be other particles, only we don't really know what we're looking for. (The current dark matter candidate, referred to as the WIMP, isn't one of those 17 identified particles, for example.) That contrasts with the search for the Higgs, for instance, in that we had a pretty good idea of what the Higgs would look like, we just didn't know exactly at what energy level we'd find that signature.

The quote makes it sound like we've detected all of the particles we ever predict we'll detect. Which really isn't true. A better characterization would be that "with the discovery of the Higgs boson, all of the particles that are well-described by the existing theory have been detected, but other particles are hypothesized."

- The standard model predicts 17 elementary particles, but also a lot of composite particles such as hadrons and mesons.

I think the quoted sentence referred to what has been predicted from existing well tested theory, as you seem to imply.

And it is a big deal, in that it predicts all of everyday physics, the basics for the bulk of physics up to 100's of GeV, while:

- our daily chemistry happens at a few eV.- the opto-electric properties of materials from their valence electrons happens because effectively light electrons and holes resides at a few keV.- atmospheric discharges and gamma ray flashes of positive "dark lightning" happens at 10s - 100s of keV.- nuclear fusion happens at 10's - 100's of MeV

Oh, we will be affected by the happenstance high energy cosmic ray and its products too, as well as likely a few humans or their immediate environment will be heated by a minuscule amount by having a nucleus hit dead on by a dark matter particle every year.

But if it isn't already a chemical phenomena, an opto-electronic phenomena, an atmospheric discharge phenomena, or solar fusion, we can forget about its daily effects.

But yes, it is misleading on particle physics.

* IIRC, it is something like 1-2 sigma on seeing that the particle spin is 0 instead of the desired 3 sigma.

And those bulk particles are created and annihilated all the time, though from Strassler's other writings I don't think they should be confused with virtual particles. Virtual particles wouldn't have real mass and energies, right?

My non-expert understanding of virtual particles is that they would be expected to appear in open space, not tightly bound inside a hadron.

A virtual particle is a particle-antiparticle pair that appears "out of nowhere" (energy spontaneously converted into mass) sent off in opposite directions, but not with enough force to escape each other's electrical pull. The two fall back into each other and annihilate. The energy released during annihilation is basically equal to the amount of energy used to create the particles in the first place.

Why would such a thing happen? Random interactions of energy in a vacuum. Think of a pool with lots of random ripples on the surface. Once in a while, a set of ripples will hit each other just right to send a drop up in the air. This is not the rest state for that bit of water (and for the moment, the pool has mysteriously "lost" some of its water), but everything goes back to normal after a second when the drop falls back into the surface. Once in a great while, the drop lands outside the pool and the pool's status is permanently changed just a little (Hawking radiation).

Inside a hadron with "zillions" of quark-antiquark pairs, this is not so much random drops from pool ripples and more like the spray from turbulence in a directed flow. Depending on how far one is willing to stretch the analogy, the spray that lands outside the main body of water might be responsible for particle decay.

Disclaimer: This is all based on my intuitive understanding of things discovered after I was done with formal schooling. Back then, they were still telling us a neutron was a proton and electron stuck together.

With the discovery of the Higgs boson, the predicted collection of fundamental particles was complete.

I hate that characterization of the current state of particle physics. It's only sort of true, and it's misleading to the layman.

The standard model predicts 17 particles. We have now actually seen evidence of all 17 in detectors. But we know the standard model is incomplete. We're pretty much 99.999% sure there must be other particles, only we don't really know what we're looking for. (The current dark matter candidate, referred to as the WIMP, isn't one of those 17 identified particles, for example.) That contrasts with the search for the Higgs, for instance, in that we had a pretty good idea of what the Higgs would look like, we just didn't know exactly at what energy level we'd find that signature.

The quote makes it sound like we've detected all of the particles we ever predict we'll detect. Which really isn't true. A better characterization would be that "with the discovery of the Higgs boson, all of the particles that are well-described by the existing theory have been detected, but other particles are hypothesized."

- The standard model predicts 17 elementary particles, but also a lot of composite particles such as hadrons and mesons.

I think the quoted sentence referred to what has been predicted from existing well tested theory, as you seem to imply.

And it is a big deal, in that it predicts all of everyday physics, the basics for the bulk of physics up to 100's of GeV, while:

- our daily chemistry happens at a few eV.- the opto-electric properties of materials from their valence electrons happens because effectively light electrons and holes resides at a few keV.- atmospheric discharges and gamma ray flashes of positive "dark lightning" happens at 10s - 100s of keV.- nuclear fusion happens at 10's - 100's of MeV

Oh, we will be affected by the happenstance high energy cosmic ray and its products too, as well as likely a few humans or their immediate environment will be heated by a minuscule amount by having a nucleus hit dead on by a dark matter particle every year.

But if it isn't already a chemical phenomena, an opto-electronic phenomena, an atmospheric discharge phenomena, or solar fusion, we can forget about its daily effects.

But yes, it is misleading on particle physics.

* IIRC, it is something like 1-2 sigma on seeing that the particle spin is 0 instead of the desired 3 sigma.

I didn't say it wasn't a big deal. I just said that the statement made in the article makes it sound like we wouldn't have expected to find any more types particles. We fully expect to find more types of particles.

Makes sense (especially back when we thought that protons were elementary particles), but how does electron capture turn an up quark into a down quark???

Yeah, that's really weird. It's how the weak force works, actually. The gluons carrying the strong force can instigate a change in color, but the W and Z bosons that carry the weak force can cause a change in flavor, turning any up-type quark into a down-type quark and vice versa. And yes, that can change protons into neutrons, and vice versa, making atoms change what element they are. Which is totally nuts.

But if it didn't happen, we wouldn't have radiocarbon dating. Or the thermonuclear reactions that power the sun. So it's kinda useful.

The standard model predicts 17 particles. We have now actually seen evidence of all 17 in detectors. But we know the standard model is incomplete. We're pretty much 99.999% sure there must be other particles, only we don't really know what we're looking for.

Which is the wonderfully ironic thing about particle physics these days - we only know how to discover the things we're already sure we know about.

Everyone's waiting for the Higgs announcement. When it comes everyone will slap backs, hand out a few nobels, and tell the press about what a great advancement it is.

But if you think about it, we already know everything about the Higgs we could ever want to. All we really don't know is its mass, which is ironic in itself. If this is what we get, money wasted.

Fermi spent the last 10 years adding decimals to the end of the top quark mass. And the useful output of that is pretty much exactly zero. Let's hope we get more from LHC.

Damn you all... I'm a few episodes away from the end of the new Arrested Development season and wondering what to watch next.

With all the Quark/DS9 OT-ness here, I'm seriously considering running through DS9 again...

or you could read Finnegans Wake!maybe not

I've heard it said that Finnegans Wake is so dense with obscure references that the only person who could understand it would be James Joyce himself... I don't think I'll even attempt reading that one myself.